1. Field of the Invention
The present invention relates to a lithographic apparatus and a positioning apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The lithographic apparatus according to the state of the art may include a substrate table which is constructed to hold a substrate. Whether usage is made of the so-called stepper or the so-called scanner, in each case it is required that the substrate table is movable over a relatively large distance, e.g. to irradiate different part of the substrate, to perform measurements on different parts of the substrate, to exchange the substrate, etc. Due to high accuracy which is required for the pattern which is to be brought onto the substrate in any way, a high positioning accuracy of the substrate is required. To achieve an accurate positioning, The lithographic apparatus includes a measurement system to measure a position of the substrate table, e.g. with respect to a reference structure such as the metrology frame which may also serve as a reference for a projection system of the lithographic apparatus. Due to their inherent high accuracy's, interferometers may be used in the measurement system to measure the position of the substrate table. For this purpose, several types of interferometers are known, an example being described in Agilent's Laser and Optics User's Manual (p/n 05517-90045). The measurement system may detect the position of the substrate table in e.g. a two dimensional plane, however commonly several other degrees of freedom are measured too, including e.g. a rotation of the substrate table, a height with respect to the two dimensional plane, etc. Due to the large range of movement which is required for the substrate table as has been described above, a length of a measurement beam of an interferometer comprised in the measurement system may very significantly depending on a position of the substrate table. If the substrate table is at its current position relatively close to a particular interferometer, then a beam length of that particular interferometer will be relatively short, while if the substrate table is at another instance at a position relatively remote from that particular interferometer, then the beam length of the measurement beam of that particular interferometer might be relatively long. In a practical implementation, an interferometer beam length may extend up to a distance in an order of magnitude of e.g. 0,5 meters. The possibly large and varying interferometer beam length results in a deterioration of a measurement accuracy of the interferometer. As known, a read out of an interferometer is dependent on a wavelength of the interferometer beam. This wavelength in turn depends on a plurality of physical quantities, amongst which a temperature, an atmospheric pressure of a gas through which the interferometer beam travels, etc. Due to all kind of disturbance factors, such as a movement of the substrate table, movements of any other movable part of the lithographic apparatus, generation of heat, airflow, etc, variations in such physical parameters which have an influence on the wavelength of the interferometer beam may occur, thus affecting or locally affecting a wavelength of the interferometer beam. Therefor, an accuracy of the measurement system is limited by these factors, which may result in a limitation in an accuracy to position the substrate table, which may in turn lead to a limitation in an accuracy in applying the pattern on the substrate.